What information is missing in the "field line"
diagram that was given by the field vector diagram?

c)    F = 16000 N

Explanation:

For this exercise we use Newton's second law

         F = ma

they tell us that adding the other wagons the acceleration of the locomotive must be maintained

      F = m a

by adding the other four wagons

mass = 4 no

therefore to maintain the force you must also raise the same factor

         Fe = 4Fo

         Fe = 4 4000

         F = 16000 N